LeetCode 200. Number of Islands

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

DFS method.

#include <string>
#include <vector>
#include <iostream>
using namespace std;
/*
    11110
    11010
    11000
    00000  ->Island = 1

    11000
    11000
    00100
    00011  ->Island = 3
*/

void flipBoard(vector< vector<char> >& grid, int i, int j, vector< vector<bool> >& used) {
    grid[i][j] = '0';
    vector< vector<int> > dir {
        {0, 1},
        {0, -1},
        {-1, 0},
        {1, 0}};
    for(int k = 0; k < 4; ++k) {
        int tx = dir[k][0] + i;
        int ty = dir[k][1] + j;
        if(tx >= 0 && tx < grid.size() && ty >= 0 && ty < grid[i].size() && grid[tx][ty] == '1' && !used[tx][ty]) {
            used[tx][ty] = true;
            flipBoard(grid, tx, ty, used);
        }
    }
}
int numIslands(vector< vector<char> >& grid) {
    if(grid.size() == 0 || grid[0].size() == 0) return 0;
    int m = grid.size();
    int n = grid[0].size();
    vector< vector<bool> > used(m, vector<bool>(n, false));
    int number = 0;
    for(int i = 0; i < m; ++i) {
        for(int j = 0; j < n; ++j) {
            if(grid[i][j] == '1' && !used[i][j]) {
                used[i][j] = true;
                flipBoard(grid, i, j, used);
                number++;
            }
        }
    }
    return number;
}

int main(void) {
    vector< vector<char> > matrix {
        {'1', '1', '0', '0', '0'},
        {'1', '1', '1', '0', '0'},
        {'0', '0', '1', '0', '0'},
        {'0', '0', '0', '1', '1'}};
    int num = numIslands(matrix);
    cout << num << endl;
}